A Most Subtle Eclipse of the Moon

Lunar Eclipse
Lunar Eclipse of October 27, 2004 photo by Curtis Roelle

This month there is an eclipse of the moon that you might not notice without prior knowledge that it’s happening. It’s not a total lunar eclipse; it’s not even a partial eclipse. It is what astronomers call a penumbral eclipse. The eclipse occurs on the evening of February 10.

A lunar eclipse occurs when the moon, in its orbit about the Earth passes through its shadow. Because all three bodies must be lined up with the Earth in the middle, a lunar eclipse happens at the time of full moon.

In our solar system, every celestial body, including planets, moons and asteroids, have shadows cast by the sun. The sun is not a point source of light. It has an angular size and is larger than any other solar system object. Each shadow has two concentric elements. The dark central shadow is called the umbra. The lighter outer shadow is the penumbra.

During a lunar eclipse, if the moon completely enters the Earth’s umbra, then the eclipse is total. If only part of the moon enters the umbra while the rest of it remains in the penumbra, then the eclipse is partial.

For this month’s eclipse, the moon enters only into the penumbral shadow and completely misses the umbra. For that reason, this third type of lunar eclipse is called a penumbral lunar eclipse.

The event is favorable for our geographical area because it occurs early in the evening on a Friday night, February 10.

When the moon first touches the penumbra at 5:32 p.m. EST. In Westminster, the moon rises one minute later. In about 45 minutes, with the moon higher in the sky, there should be a noticeable darkening along its top marking the beginning of the entry into the penumbral shadow.

Mid eclipse occurs at 7:44 p.m. By this time the left-hand side of the moon may be considerably darker than the right. That is because the former is deeper into the penumbra, approaching the dark umbra.

As the moon exits the penumbra, the brightness increases. It may cease to be noticeable about 3 hours after it first became visible, or after 9 p.m. The penumbral lunar eclipse ends at 9:55 p.m.

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